The subject matter of the present invention pertains to means for producing a controlled current through an inductance; in particular, the inductive yoke of a cathode-ray tube electromagnetic deflection system.
As is known to the art, the presentation of a raster-scan image on the display screen of a cathode-ray tube requires that an electron beam be deflected across the screen in two mutually orthogonal directions; a first direction at a rate of once per image field, and a second direction at a rate of once per raster line. At the completion of each field or line, the beam, usually blanked to prevent visual interference with the displayed image, is deflected back, or retraced, across the display screen to a starting point for the next field or line. Many circuits have been devised for deflecting the beam back to a new starting point in the least amount of time consistent with normal limits of power availability and component power rating. Examples of such circuits are well known to the art and, for horizontal retrace scanning, include those described in Morrey U.S. Pat. No. 3,739,224, Wilcox U.S. Pat. No. 3,727,096, Rodal U.S. Pat. No. 3,529,206, Wheatley U.S. Pat. No. 3,449,622, Ushikubo et al U.S. Pat. No. 3,444,424, Hansen et al U.S. Pat. No. 3,430,096, and Jones U.S. Pat. No. 3,428,856.
Recently, it has become evident that rapid retrace can be achieved most efficiently by use of a dual-mode amplifier capable of being switched between a linear mode for forward scanning operation, and a non-linear resonant mode for retrace operation. Examples of such circuits include those disclosed in Hilburn U.S. Pat. No. 3,786,303 and Spencer U.S. Pat. No. 3,816,792. In the Hilburn circuit, a fly-back capacitor is connected in parallel with the horizontal deflection coil and a transistor connecting the capacitor and the coil to a source of negative potential is switched, under control of a synchronization signal separate from the input deflection signal, to perform the desired intermodal alternation. The Spencer circuit is similar except that the fly-back capacitor is switched, also under control of a separate synchronization signal, to be in the circuit only during retrace operation.
A primary disadvantage of both the Hilburn circuit and the Spencer circuit is that they require the provision of a synchronization signal in addition to the conventional input deflection signal, and they require the inclusion of specialized switching circuitry to perform the intermodal alternation. An additional disadvantage is that neither circuit is capable of processing an input deflection signal of other than the conventional positive-going forward scan/negative-going reverse retrace form.